# combine videos using zsh
for file in *.mp4; do
    echo "file '$file'" >> filelist.txt
done
ffmpeg -f concat -i filelist.txt -c copy output.mp4

# extract audio from video
ffmpeg -i example.mp4 -acodec pcm_s16le -ar 16000 -ac 1 -q:a 0 -map a output.wav
# encode a video
ffmpeg -i input_video.mp4 -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 128k output_video.mp4